prime meridian

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A fixed circle of longitude on the surface of a body, used as a reference from which other longitudes can be measured west or east. The prime meridian on Earth passes through the transit circle established by G. B. Airy at Greenwich, and defines the zero of terrestrial longitude. On other bodies the prime meridian is defined with respect to a known surface feature, such as a small crater.

Subjects: Maritime History — Astronomy and Astrophysics.

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